The present invention relates to a glow plug, particularly of the kind for use in a diesel engine.
Hereinafter, the term “front” refers to a heating end side with respect to the axial direction of a glow plug, and the term “rear” refers to a side opposite the front side.
FIG. 7 shows a conventional-type glow plug 100 that includes a ceramic heater 111 provided with an insulating ceramic substrate 193, a heating element 191 and a pair of electric conductors 195 and 197, a cylindrical metallic shell 113, a metallic sleeve 115, a rod-shaped electrode 117 and a lead wire 199. The ceramic heater 111 is fixed in a front end portion 113a of the metallic shell 113 by brazing the metallic sleeve 115 onto the ceramic heater 111 and then brazing the metallic shell 113 onto the metallic sleeve 115. On the other hand, the electrode 117 is fixed in a rear portion of the metallic shell 113 by fitting a ring-shaped glass sealant 121 with a bushing 123 and a securing ring 125. The heating element 191 is embedded in the ceramic substrate 193, and the electric conductors 195 and 197 connect the heating element 191 to the metallic shell 113 and the electrode 117 via the metallic sleeve 115 and the lead wire 199, respectively. Further, a terminal element 119 is fitted on the electrode 117 for connection to an external power source (not shown).
The electrode 117 is generally made of an iron material with rigidity and has a relatively large diameter (of the order of several millimeters) so as to provide not only excellent electrical properties but also resistance to bending. The electrode 117 considerably increases in weight with increase in its length. It is how ver possible to increase the length of the glow plug 100 by producing both the metallic shell 113 and the electrode 117 in increased lengths, so that the glow plug 100 is suitable for use in a direct-injection engine.